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Abstract

Silicon (Si) is the most abundant element present in the Earth's crust besides oxygen.
However, the exact biological roles of silicon remain unknown. Moreover, the ortho-silicic
acid (H4SiO4), as a major form of bioavailable silicon for both humans and animals, has not been
given adequate attention so far. Silicon has already been associated with bone mineralization,
collagen synthesis, skin, hair and nails health atherosclerosis, Alzheimer disease,
immune system enhancement, and with some other disorders or pharmacological effects.
Beside the ortho-silicic acid and its stabilized formulations such as choline chloride-stabilized
ortho-silicic acid and sodium or potassium silicates (e.g. M2SiO3; M= Na,K), the most important sources that release ortho-silicic acid as a bioavailable
form of silicon are: colloidal silicic acid (hydrated silica gel), silica gel (amorphous
silicon dioxide), and zeolites. Although all these compounds are characterized by
substantial water insolubility, they release small, but significant, equilibrium concentration
of ortho-silicic acid (H4SiO4) in contact with water and physiological fluids. Even though certain pharmacological
effects of these compounds might be attributed to specific structural characteristics
that result in profound adsorption and absorption properties, they all exhibit similar
pharmacological profiles readily comparable to ortho-silicic acid effects. The most
unusual ortho-silicic acid-releasing agents are certain types of zeolites, a class
of aluminosilicates with well described ion(cation)-exchange properties. Numerous
biological activities of some types of zeolites documented so far might probably be
attributable to the ortho-silicic acid-releasing property. In this review, we therefore
discuss biological and potential therapeutic effects of ortho-silicic acid and ortho-silicic
acid -releasing silicon compounds as its major natural sources.

Keywords:

Introduction

Silicon (Si) is the most abundant element (27.2%) present in the earth's crust following
oxygen (45.5%) [1]. Silicon is known for a number of important chemical and physical properties, i.e. semiconductor property that are used in various scientific and technical applications.
These Si features, along with structural complexity of its compounds, have attracted
researchers from the earliest times [2]. In particular, silicon dioxide or silica (SiO2) is the most studied chemical compound following water, and the most important Si-containing
inorganic substance [1]. Formally, silica (SiO2) is a silicic acid anhydride of monomeric ortho-silicic acid (H4SiO4), which is water soluble and stable in highly diluted aqueous solutions. Moreover,
several “lower” hydrated forms of ortho-silicic acid exist in aqueous solutions as
well including meta-silicic acid (H2SiO3 or lower oligomers like di-silicic (H2Si2O5) and tri-silicic acids (H2Si3O7) including their hydrated forms pentahydro-silicic (H10Si2O9), and pyro-silicic acids (H6Si2O7) [1]. These are water soluble, formed in reversible equilibrium reactions from H4SiO4 and stable in diluted aqueous solutions. During a prolonged storage period, at increased
concentration or in an acidic environment, these low molecular silicic acids undergo
further condensation by cross-linking and dehydration. This process results in formation
of poly-silicic acids chains of variable composition [SiOx(OH)4-2x and complex structure [1]. The end product is a jelly-like precipitate, namely hydrated silica (SiO2·xH2O; often referred as “colloidal silicic acid” or “hydrated silica gel”). Further condensation
follows which is accompanied by dehydration yielding less hydrated silicon dioxide
(SiO2) phases, also known as “silica gel” or “amorphous silicon dioxide”.

Lower molecular forms, especially the ortho-silicic acid (H4SiO4; Figure 1), play a crucial role in delivering silicon to the living organisms’ cells and thus
represent major sources of silicon for both humans and animals. Most of the silica
in aqueous systems and oceans is available in the form of H4SiO4, which makes it an important compound in environmental silicon-chemistry and biology
[3]. In this paper, we critically review the most recent findings on biological effects
of Si and ortho-silicic acid on animals and human beings. Moreover, we propose that
previously observed positive biological effects of various colloidal silicic acids
(various hydrated silica gels) as well as some zeolites [4-6], e.g. zeolite A (Figure 2) and clinoptilolite (Figure 3), might be, at least partially, ascribed to the ortho-silicic acid-releasing property.

Figure 2.Zeolite A structure: an assembly of framework's cages (tiles). Centre of a tile is the centre of a void in the framework. Voids are connected with
adjacent ones through the large "windows" which are faces of tiles.

Silicon represents the third most abundant trace element in the human body [7,8]. For example, it is present in 1–10 parts-per-million (ppm) in hair [9], nails [10], in the cornfield epidermis, and in the epicuticle of hair [11,12]. Silicon is naturally present in food as a silicon dioxide (SiO2), free ortho-silicic acid (H4SiO4), silicic acids bounded to certain nutrients, and in the silicate form. Although
silicon is a life-important micronutrient mineral, in our opinion it has not received
adequate attention. Considering the abundance of silicon, both in the nature and humans,
it is expected that it should play an important role in human and animal health.

Silicon bioavailability and consumption

Presently, many biological roles of silicon remain unknown [13]. Consequently, the recommended daily silicon intake (RDI) has not yet been set [13,14]. Considering the risk assessment of amorphous silicon dioxide as common silicon source
(e.g. food additive E551), the safe upper intake level (UIL) may be estimated as 700
mg/day for adults, that is the equivalent to 12 mg silicon/kg bw/day for a 60 kg adult
[15]. These numbers refer to the amorphous silicon dioxide form and only small amounts
of silicon (as H4SiO4) are actually released in the gastrointestinal (GI) tract and subsequently absorbed
in the systemic circulation. Due to lack of data, it is difficult to set a recommended
upper intake level for silicon. Moreover, little information on the intake of dietary
silicon by humans is available. A mean intake of daily silicon has been reported in
Finland [16], (29 mg silicon/day) and in a typical British diet (20–50 mg silicon/day) [17-19]. This corresponds to 0.3-0.8 mg/silicon/kg bw/day for a 60 kg person. These data
are in the same range as the estimated mean intakes of silicon in the USA (30 and
33 mg silicon/day in men, and 24 and 25 mg silicon/day in women, respectively) [8]. Silicon intake decreases with age to less than 20 mg silicon/day (18.6 ± 4.6 mg
silicon/day for elderly British woman in an unrelated randomised controlled intervention
study) [20].

Generally, silicon is abundantly present in foods derived from plants such as: cereals,
oats, barley, white wheat flour, and polished rice. In contrast, silicon levels are
lower in animal foods including meat or dairy products. Furthermore, silicon is present
in drinking waters, mineral waters, and in beer as well [17]. However, Jugdaohsingh et al. [21] raised some doubt on utilisation of silicon from drinking water in an animal rat
study as no significant differences were found in the silicon bone concentration when
the drinking water was supplemented with silicon in the ortho-silicic acid form. Indeed,
the major sources of silicon in the typical Western hemisphere diet comes from cereals
(30%), followed by fruits, beverages and vegetables, which altogether comprise around
75% of total silicon intake [20]. Even though plant food contains high levels of silicon, its bioavailability from
these sources is questionable, due to poor solubility of actual silicon forms present
in these foods [18,19,22]. Efficient absorption in the GI tract would require their breakdown to soluble species
such as ortho-silicic acid, present in drinking and mineral waters in the range of
2 to 5 mg silicon/L [23] and in beer ranging from 9 to 39 mg silicon/L [18,24]. Absorption studies indicate that the ortho-silicic acid is a main readily bioavailable
source of silicon for humans, whereas its higher polymers are not of significant absorbability
[25]. In a placebo-controlled study on eight volunteers, Jugdaohsingh et al. [25] showed that 53% of administered ortho-silicic acid is excreted in the urine, whereas
the ingestion of polymeric silicic acid causes only a marginal increase of silicon
in the urine. This result substantiates the statement that polymeric silicic acids
and amorphous silicon dioxide are of poor bioavailability.

Besides the ortho-silicic acid, water soluble silicates are bioavailable silicon forms
as well. For instance, pharmaceutically acceptable alkali metals silicates (M2SiO3; M= Na, K) in adequately diluted aqueous solutions, release ortho-silicic acid (H4SiO4) upon contact with stomach hydrochloric acid (HCl). Popplewell et al. [26] employed a tracer dose of radiolabelled ammonium silicate to measure total uptake
and urine excretion. Their results revealed that 36% of ingested dose was absorbed
and completely excreted in urine within 48h. However, elimination occurred in two
steps where the major dose (90%) has been excreted within the first 2.7 hours. They
suggested that excess silicon is eliminated from the body through two distinct processes,
differing significantly in the duration. The ‘slower process’ is thought to include
the intracellular uptake and release of silicon, whilst the ‘faster process’ probably
includes retention of silicon in the extracellular fluids [26]. These data report on increased silicon levels in serum upon consumption of silicon-rich
food [7,27], showing that at least some silicon is available from food as well. Indeed, selective
silicon deprivation in rats showed a significant drop of urinary silicon excretion
and fasting silicon serum concentration, suggesting that the rats actively regulate
silicon levels via urinary conservation, perhaps through renal re-absorption [21]. Most of silicon present in the serum is filtered by the kidney [7,28] suggesting the kidney as its major excretion route; silicon levels in serum correlate
with those in urine. However, it is still not clear how and if the body can efficiently
retain adequate doses of silicon.

In concentrated solutions, ortho-silicic acid (H4SiO4) has to be stabilized to avoid its polymerization into poly-silicic acids and eventually
into silica gel, resulting in a decreased silicon bioavailability. This issue has
been solved in the field of pharmaceutical technology by use of choline chloride in
aqueous glycerol solution. This resulted in development of a liquid formulation known
as choline-stabilized ortho-silicic acid (ch-OSA). Choline chloride-stabilized ortho-silicic
acid is not a new chemical entity of ortho-silicic acid, but a complex of H4SiO4 and choline chloride formed by several possible hydrogen bonds between these two
compounds. Subsequently, from the standpoint of nutrition and pharmacology, the effects
of ch-OSA must involve effects of both H4SiO4 and choline chloride rather than a new chemical entity. Due to a possible impact
of choline chloride on the chemical stability of H4SiO4, certain specific biological effects different from those of a pure ortho-silicic
acid or its immediate releasing compounds (e.g. sodium silicate), must be taken in
account. Ch-OSA has been approved for human consumption and is known to be non-toxic.
Its lethal doses (LD) exceed 5000 mg/kg bw in humans [29] and 6640 mg/kg bw in animals [30]. The ch-OSA represents the most bioavailable source of silicon [22,29]. Moreover, in a randomized placebo-controlled study [29], the bioavailability of ch-OSA during maternal transfer to the offspring was investigated
in a supplementation study with pigs. The authors correlated significantly higher
silicon concentrations in the serum of weanling piglets from supplemented sows and
maternal transfer of absorbed silicon between sows and their offspring during lactation
with high bioavailability of silicon from ch-OSA. Importantly, highly bioavailable
silicon from ch-OSA did not altered calcium, phosphorus and magnesium levels in blood.

It was reported that silicon is connected with bone mineralization and osteoporosis
[31], collagen synthesis and ageing of skin [11], condition of hair and nails [32], atherosclerosis [33,34], Alzheimer disease [9,35,36], as well as with other biological effects and disorders. Trace minerals are known
to generally play a vital role in the human body homeostasis [37] and the serum levels of silicon are similar to other trace elements, i.e. of iron, copper, and zinc [38]. Silicon is excreted through the urine in similar orders of magnitude as calcium.
Some researches claim that silicon does not act as a protein-bounding element in plasma
and is believed to exist almost entirely as un-dissociated monomeric ortho-silicic
acid [28]. While early analyses showed that serum contains 50–60 μg silicon/dL [38,39], more recent analyses indicate that human serum contains 11–25 μg silicon/dL, or
levels ranging between 24 and 31 μg/dL (8.5 and 11.1 μmol/L), detected by absorption
spectrometry in large population groups [40]. Interestingly, pregnant women had very low serum silicon concentrations (3.3-4.3
μg/dL) in comparison with infants that have high concentrations between 34 and 69
μg/dL [27,41]. Moreover, silicon concentrations in serum showed a statistically significant age
and sex dependency, as it seems that silicon concentrations decrease with age, especially
in woman [40].

Biological importance of silicon might be analysed in the context of its bio-distribution
in the body. For example, the highest silicon concentration has been measured in connective
tissues, especially in the aorta, tracheas, bone, and skin. Low levels of silicon
in the form of ortho-silicic acid [42-44] may be found in liver, heart, muscle, and lung [45]. It is therefore plausible to assume that observed decrease of silicon concentration
in the ageing population may be linked to several degenerative disorders, including
atherosclerosis. Supplementation of the regular diet with bioavailable forms of silicon
may therefore have a therapeutic potential including prevention of degenerative processes.
Several experiments have already confirmed this hypothesis. For example, in a controlled
animal study, spontaneously hypertensive rats had lower blood pressure upon supplementation
with soluble silicon [44], whilst silicon deficiency in animals has been found to be connected with bone defects
and impaired synthesis of connective tissue compounds, such as collagen and glycosaminoglycans
[46-48]. It is therefore reasonable to assume that silicon deficiency or lower bioavailability
may be linked to problems with bone structure and collagen production. Moreover, silicon
was shown to be uniquely localized in active growth areas in young bones of animals
where a close relationship between silicon concentration and the degree of mineralization
has been assessed [46,49]. Studies confirmed the essential role of silicon in the growth and skeletal development
of chicks that during silicon deprivation showed significantly retarded skeletal development
[50]. Experimental silicon deprivation in rats [51-53] and chicks [46,47] demonstrated striking effects on skeletal growth and bone metabolism as well. On
the other hand, the controlled animal study of Jugdaohsingh et al. [21] showed no profound effects of a silicon-deficient diet on the bone growth and skeletal
development in rats. Silicon concentrations in the tibia and soft tissues did not
differ from those in rats on a silicon-deficient diet where the silicon was supplemented
in drinking water. Nevertheless, silicon levels in tibia were much lower compared
to the reference group fed by a silicon rich diet. Body and bone lengths were also
found to be lower in comparison with the reference group, while reduction in bone
growth plate thickness was found in silicon deprived rats [21].

Moreover, Reffit et al. [54] found that ortho-silicic acid stimulates collagen type 1 synthesis in human osteoblast-like
cells and skin fibroblasts and enhances osteoblastic differentiation in the MG-63
cells in vitro. Ortho-silicic acid did not alter collagen type 1 gene expression, but it modulated
the activity of prolyl hydroxylase, an enzyme involved in the production of collagen
[55]. Similarly, Schütze et al. [56] reported that the zeolite A stimulated DNA synthesis in osteoblasts and inhibited
osteoclast-mediated bone resorption in vitro. This is probably attributable to the ortho-silicic acid-releasing property of zeolite
A.

The mechanism underlying observed biological effects of silicon may probably be ascribed
to its interrelationships with other elements present in the body such as molybdenum
[57] aluminium [9,35,58,59], and calcium [46,49,50]. For instance, it was proven that silicon levels are strongly affected by molybdenum
intake, and vice versa[59]. Furthermore, silicon accelerates the rate of bone mineralization and calcification
as shown in controlled animal studies, in a similar manner that was demonstrated for
vitamin D [11,50]. It is well known that vitamin D increases the rate of bone mineralization and bone
formation [60], and that its deficiency leads to less mature bone development. Vitamin D is known
to be important in calcium metabolism, but silicon-deficient cockerels’ skulls in
a controlled animal study showed lower calcification and collagen levels irrespective
of the vitamin D dietary levels suggesting a vitamin D-independent mechanism of action
[61]. Jugdaohsingh et al. [21] found that silicon supplementation in drinking water did not significantly altered
silicon concentrations in bones and suggested that some other nutritional co-factor
is required for maximal silicon uptake into bone and that this co-factor was absent
in rats fed with a low-silicon diet compared to the reference group fed by a silicon-rich
diet. They suggested vitamin K as such co-factor, which is important in bone mineralisation
through carboxylation of osteocalcin, and whose deficiency might influence incorporation
of minerals such as silicon in the bones.

Osteoporosis

Osteoporosis is among leading causes of morbidity and mortality worldwide [62]. It is defined as a progressive skeletal disorder, characterised by low bone mass
(osteopenia) and micro-architectural deterioration [63]. Interestingly, the administration of silicon in a controlled clinical study induced
a significant increase in femoral bone mineral density in osteoporotic women [31]. Direct relationship between silicon content and bone formation has been shown by
Moukarzel et al. [64]. They found a correlation between decreased silicon concentrations in total parenterally
fed infants with a decreased bone mineral content. This was the first observation
of a possible dietary deficiency of silicon in humans. A randomized controlled animal
study on aged ovariectomized rats revealed that long-term preventive treatment with
ch-OSA prevented partial femoral bone loss and had a positive effect on the bone turnover
[65]. Dietary silicon is associated with postmenopausal bone turnover and bone mineral
density at the women's age when the risk of osteoporosis increases. Moreover, in a
cohort study on 3198 middle-aged woman (50–62 years) it was shown that silicon interacts
with the oestrogen status on bone mineral density, suggesting that oestrogen status
is important for the silicon metabolism in bone health [66].

Skin and hair

Typical sign of ageing skin is fall off of silicon and hyaluronic acid levels in connective
tissues. This results in loss of moisture and elasticity in the skin. Appearance of
hair and nails can also be affected by lower silicon levels, since they are basically
composed of keratin proteins. As previously discussed, ortho-silicic acid may stimulate
collagen production and connective tissue function and repair. For example, Barel
et al. [67] conducted experiments on females, aged between 40–65 years, with clear clinical signs
of photo-ageing of facial skin. Their randomized double-blinded placebo-controlled
study illustrates positive effects of ch-OSA taken as an oral supplement on skin micro
relief and skin anisotropy in woman with photo-aged skin. Skin roughness and the difference
in longitudinal and lateral shear propagation time decreased in the ch-OSA group,
suggesting improvement in isotropy of the skin. In addition, ch-OSA intake positively
affected the brittleness of hair and nails. Oral supplementation with ch-OSA had positive
effects on hair morphology and tensile strengths, as shown in a randomized placebo-controlled
double blind study by Wickett et al. [68].

Alzheimer disease

Aluminium (as Al3+ ion) is a well-known neurotoxin. Aluminium salts may accelerate oxidative damage
of biomolecules. Importantly, it has been detected in neurons bearing neurofibrillary
tangles in Alzheimer's and Parkinson's disease with dementia as shown in controlled
studies [69,70]. Amorphous aluminosilicates have been found at the core of senile plaques in Alzheimer's
disease [69,71], and have consequently been implicated as one of the possible causal factors that
contribute to Alzheimer’s disease. Since aluminosilicates are water insoluble compounds,
the transport path to the brain is still not well understood. By reducing the bioavailability
of aluminium, it may be possible to limit its neurotoxicity. Consumption of moderately
high amounts of beer in humans and ortho-silicic acid in animals has shown to reduce
aluminium uptake from the digestive tract and slow down the accumulation of this metal
in the brain tissue [36,72]. Silicic acid has also been found to induce down-regulation of endogenous antioxidant
enzymes associated with aluminium administration and to normalize tumour necrosis
factor alpha (TNFα) mRNA expression [35]. Although the effect of silicic acid on aluminium absorption and excretion from human
body produced conflicting results so far as shown in an open-label clinical study
[7], in a controlled clinical study it was shown that silicic acid substantially reduces
aluminium bioavailability to humans [73]. In fact, it was already found that silicon reduces the aluminium toxicity and absorption
in some plants and animals that belong to different biological systems [74-76]. This is possible as silicon competes with aluminium in biological systems such as
fresh water, as suggested by Birchall and Chappell study perfomed on the geochemical
ground [77], and later confirmed by Taylor et al. in randomized double blind study [78]. They found that soft water contains less silicic acid and more aluminium, while
hard waters contain more silicic acid and less aluminium.

Removal of aluminium from the body and its reduced absorption by simultaneous administration
of silicic acid was tested and proven by Exley et al. in controlled clinical study
[59]. They showed reduced urinary excretion of aluminium along with unaltered urinary
excretion of trace elements such as iron in persons to whom silicic acid-rich mineral
water was administered. Moreover, they documented that regular drinking of a silicon-rich
mineral water during a period of 3 months significantly reduced the body burden of
aluminium. Similar results were obtained by Davenward et al. [79] who showed that silicon-rich mineral waters can be used as a non-invasive method
to reduce the body burden of aluminium in both Alzheimer's patients and control group
by facilitating the removal of aluminium via the urine without any concomitant effect. They also showed clinically relevant improvements
of cognitive performances in at least 3 out of 15 individuals with Alzheimer disease.
This implies a possible use of ortho-silicic acid as long-term non-invasive therapy
for reduction of aluminium in Alzheimer's disease patients. The mechanism through
which aluminium bioavailability reduction occurs involves interaction between aluminium
species and ortho-silicic acid where highly insoluble hydroxyaluminosilicates (HAS)
forms are produced [77,80]. This process makes aluminium unavailable for absorption.

Immunostimulatory effects

Quartz as a form of crystalline silicon dioxide has been connected with severe negative
biological effects. However, in controlled studies on mouse and rats it was shown
that sub-chronic and short-term exposure to this compound can actually have beneficial
effects on respiratory defence mechanisms by stimulating immune system through the
increase of neutrophils, T lymphocytes and NK cells. It also activates phagocytes
and consequently additional ROS production [81-83] which can help the pulmonary clearance of infectious agents. In rats, crystalline
silica caused proliferation and activation of CD8+ T cells and, to a lesser amount, of CD4+ T cells.

Recently, an “anionic alkali mineral complex” Barodon® has shown immunostimulatory
effects in horses [84], pigs [85] and other animals. Barodon® is a mixture of sodium silicate (M2SiO3, M= Na,K) and certain metal salts in an alkaline solution (pH= 13.5), where sodium-silicate
(sodium water glass) represents 60% of the total content. In a placebo-controlled
experiment in pigs, the immunostimulatory effect of Barodon® was assessed by measurement
of proliferation and activation of porcine immune cells, especially CD4+ CD8+ double-positive (dpp) T lymphocytes in peripheral blood and in the secondary lymphoid
organ [85]. As this type of T lymphocyte cells are characterized by a specific memory cell marker
CD29, they may play a role during activation of secondary immune responses as shown
in a cross-sectional and longitudinal study on pigs [86]. Moreover, Barodon® acted mainly on the lymphoid organs, implying a role in antigenic
stimulation of immune tissues [85]. Barodon® induced increased levels of MHC-II lymphocytes and non-T/non-B (N) cells
as well along with increased stimulatory mitogen activity including the activity of
PHA, concanavalin A, and pokeweed mitogen [85,87]. In a placebo-controlled experiment on pigs, it was shown that this mineral complex
exerts an adjuvant effect with hog cholera and Actinobacillus pleuropneumoniae vaccines
by increasing the antibody titres and immune cell proportions [88]. Moreover, Barodon® showed nonspecific immunostimulating effects in racing horses
and higher phagocytic activity against Staphylococcus equi subsp. equi and Staphylococcus aureus as well in a controlled study [84]. Administration of Barodon® in horse herds reduced many clinical complications, including
stress-induced respiratory disease, suggesting activation of immune cell populations
similarly to the treatment with inactivated Propionibacterium acnes[89,90]. The exact mechanism of Barodon® immunostimulatory effect is not known, although
it has been suggested that sodium silicate, the main mineral ingredient, might be
responsible for the observed immune-enhancing properties. Indeed, sodium silicate
is known to decompose quantitatively into bioavailable ortho-silicic acid (H4SiO4) in the acidic gastric juice (HCl), and as such being absorbed in the body. In this
manner, presumably all observed pharmacological effects of Barodon® are actually originated
from the ortho-silicic acid.

Pure sodium metasilicate (Na2SiO3) also bears immunostimulatory effects and acts as a potent mitochondria activator
[91]. Dietary silicon in the form of sodium metasilicate activates formation of ammonia
by elevating mitochondrial oxygen utilisation as shown in a controlled animal experiment
[91]. These findings further corroborate the hypothesis that sodium silicate might be
responsible for immunostimulatory effects of Barodon®. Once again, the pharmacologically
active species was ortho-silicic acid released upon the action of stomach hydrochlorid
acid on sodium metasilicate.

Zeolites as a source of ortho-silicic acid

Zeolites are a class of aluminosilicates of general formula (Mn+)x/n[(AlO2)x(SiO2)y·mH2O, wherein M represents a positively charged metal ion such as sodium (Na+), potassium (K+), magnesium (Mg2+), or calcium (Ca2+). Zeolites are crystalline aluminosilicates with open 3D framework structures built
of SiO4 and AlO4 tetrahedra linked to each other by sharing all the oxygen atoms to form regular intra-crystalline
cavities and channels of molecular dimensions [92]. The positively charged metal ions (e.g. Na+, K+, Ca2+, Mg2+) are positioned in these cavities of aluminosilicate skeleton which are termed as
micro- (2–20 Å), meso- (20–50 Å), and macro-(50–100 Å) -pores. These ions are readily
exchangeable in contact with aqueous solution of other positively charged ions (e.g.
heavy metal ions like Hg2+). This structural characteristic of zeolites is the base of their ion (cation)-exchange
property [93].

At present, 191 unique zeolite frameworks have been identified [94], while over 40 naturally occurring zeolite frameworks have been described. Zeolites
have been widely employed in chemical and food industries, agriculture, and environmental
technologies as adsorbents, absorbents, adsorbent filter-aids, ion-exchangers, catalysts,
active cosmetic and pharmaceutical ingredients, soil improvers, etc. [95-103]. Besides, zeolites exhibit a number of interesting biological activities [5,104,105] (Figure 4). For example, nontoxic natural zeolite clinoptilolite affects tumour cells proliferation
in vitro and might act as an adjuvant in cancer therapy [105]. Katic et al. [106] confirmed that clinoptilolite influences cell viability, cell division, and cellular
stress response that results in antiproliferative effect and apoptosis induction in vitro. Obtained results demonstrated that clinoptilolite biological effect on tumour cells
growth inhibition might be a consequence of adsorptive and ion-exchange characteristics
that cause adsorption of some serum components by clinoptilolite [106]. Similarly, clinoptilolite showed antiviral effects in vitro and a potential in antiviral therapy either for local skin application against herpesvirus
infections or oral treatment of adenovirus or enterovirus infections [107]. The antiviral mechanism is probably non-specific and is based on adsorption of viral
particles on external cavities at the clinoptilolite surface rather than a consequence
of ion-exchange properties.

Each zeolite particle acts like a large inorganic molecule and acts as a molecular
sieve with a potential in molecular medicine in molecular medicine. Their pores are
indeed, rather small (less than 2 nm to 50 nm) [108], and these structural similarities between the cages of zeolites and binding sites
of enzymes resulted in development of zeolite structures that mimic enzyme functions
[108], e.g. haemoglobin, cytochrome P450 or iron-sulphur proteins [109].

Important data on biological zeolites fate (Figure 5) and effects in vivo have been widely reported so far in the scientific literature. For example, it was
shown that zeolites bear detoxifying and decontaminant properties when added to animal
diets, reducing levels of heavy metals (e.g. lead, mercury, and cadmium) and various
organic pollutants, i.e. radionuclides (Figure 6) and antibiotics [108]. Furthermore, zeolites have been successfully utilized for haemodialysis, for cartridges
in haemoperfusions, for wound healing, and surgical incisions [108]. For instance, QuikClot and Zeomic formulations are already being marketed for haemorrhage
control [110] and dental treatment [5], respectively.

Figure 5.Structural and biochemical changes of zeolites in the digestive system (by courtesy
from Application of natural zeolites in medicine and cosmetology – ZEOMEDCOS.SWB, Baku-London, 2010).

Several toxicological studies proved that certain natural zeolite, e.g. clinoptiolite
are non-toxic and completely safe for use in human and veterinary medicine [105]. In vitro and in vivo controlled animal studies have shown that clinoptilolite is an inert substance that
may cause, in some instances, only moderate but not progressive fibrosis or mesothelioma
[111]. This effect might be attributed to side-substances present in natural zeolites,
e.g. silica or clay aluminosilicates [112]. It should be also stated that some zeolites might be extremely dangerous for human
health and exert negative biological effects. For example, erionite, a fibrous type
of natural zeolite, causes a high incidence of mesotheliomas and fibrosis in humans
and experimental animals [113].

Animal studies have also shown the possibility of zeolite A (sodium aluminosilicate)
as a viable source of silicon [4,6,114]. The latter is one of known zeolites that breaks down into bioavailable ortho-silicic
acid (H4SiO4) in the digestive system. This property arises from the structure of zeolite A which
is characterized by the same number of aluminium and silicon atoms in zeolite A [115]. Zeolite A is hydrolysed at low pH (stomach hydrochloric acid) into ortho-silicic
acid (H4SiO4) and aluminium ions (Al3+). These are combined back to the amorphous aluminosilicate. Such process readily
provides additional source of bioavailable silicon to the organism [114,116]. Indeed, randomized placebo-controlled studies on dogs [114] proved that silicon is absorbed upon oral administration of zeolite A. Comparable
results have been obtained in a randomized placebo-controlled research on horses as
well [6]. Addition of zeolite A to the diet of young racing quarter horses have resulted in
decreased skeletal injury rates and better training performance [117]. However, increased bone formation was found in randomized controlled studies on
broodmare horses [118], but not in yearling horses [119]. Food supplementation with zeolite A in calves showed no changes in bone architecture
or mechanical properties [120]. However, in a controlled study Turner et al. [120] showed increased aluminium content in the bone and cartilage of zeolite A-fed calves
which is an important safety issue for the zeolite A therapeutic usage.

Conclusion

In conclusion, we believe that ortho-silicic acid (H4SiO4) might be a prominent therapeutic agent in humans. Some potential therapeutic and
biological effects on bone formation and bone density, Alzheimer disease, immunodeficiency,
skin, hair, and nails condition, as well as on tumour growth, have already been documented
and are critically discussed in the presented paper. Acid forms of ortho-silicic acid
include: choline-chloride-stabilized ortho-silicic acid (ch-OSA) as a specific pharmaceutical
formulation of H4SiO4, simple water soluble silicate salts such as sodium silicate (E550; Na2SiO3) or potassium silicate (E560; K2SiO3), and certain water-insoluble forms that, upon contact with stomach juice (HCl),
release small, but biologically significant amounts of ortho-silicic acid. The latter
involves: colloidal silicic acid (hydrated silica gel), amorphous silicon dioxide
(E551), certain types of zeolites such as zeolite A (sodium aluminosilicate, E554;
potassium aluminosilicate, E555; calcium aluminosilicate, E556), and the natural zeolite
clinoptilolite. However, for some of the above-proposed therapeutic perspectives of
both ortho-silicic acid and ortho-silicic acid -releasing derivatives, additional
insights into biological mechanisms of action and larger studies on both animals and
humans are required.

Competing interest

The authors declare no conflict of interest.

Authors’ contributions

LMJ has prepared the body of the manuscript text and figures as well as performed
a general literature search in particular those related to animal studies. IC has
prepared the literature and manuscript parts related to orthosilic acid and zeolites
chemistry as well as interpretation of biological effects in relation to chemical
properties. SKP has prepared the literature and parts of the manuscript related to
biological effects of orthosilic acid and zeolites and performed the text revision.
KP provided the idea for the manuscript, medical interpretation of cited studies and
performed the final text revision. All authors read and approved the final manuscript.

Acknowledgements

This work was supported by the Croatian Ministry of Science, Education and Sports
(grants number 335-0982464-2393 and 335-0000000-3532).